A large variety of thin films are used in the fabrication of semiconductor devices. Current design rules for integrated circuits (ICs) demand that many features be reduced to submicron levels resulting in, for example, smaller linewidths and higher aspect ratio contact holes. These submicron size demands impact the specifications and requirements of many of the fabrication processes. The flatness of a wafer must be tightly maintained throughout the entire fabrication process in order to allow such increasingly fine and microscopic geometric features to be precisely printed. Every layer deposited on the top surface of the wafer that possesses irregularities and variations in its uniformity has an adverse affect during all subsequent processing steps that the wafer undergoes. Uniformity of the layers has become a critical factor in semiconductor wafer production. It is, therefore, essential that the wafer surface be as clean and well conditioned as is technically possible before each fabrication process in order to obtain the precision and high yields in semiconductor fabrication.
To accomplish these fabrication goals, wet process wafer cleaning and etching procedures are carried out prior to many process steps. An important change in light of these factors is a need for dramatic improvement in contamination control. Contamination control includes, for example, reducing the particulate matter present and the purity of the grade of chemicals used in IC manufacture. There is a direct correlation between device yield and performance with the purity of the chemicals used to fabricate the device. This direct correlation between defects and chemical purity spotlights the need to increase the purity of the chemicals used in all of the fabrication processes, as a means to reduce contamination.
The potential exists to decrease contamination, lower defect density and increase chemical purity over that which can be obtained by simply purchasing the highest purity grade of chemicals available. Even slight improvements in defect density, and therefore device performance, are likely to enable considerable cost savings. A significant use of chemicals in IC fabrication occurs in wet processing steps, such as wafer cleaning and wet etching processes. Wet chemical surface preparation methods have been the preferred techniques for many wafer processing methods for the last 30 years and are likely to have wide popularity and application for the foreseeable future. Wet chemical cleaning technologies are favored because of many inherent properties of aqueous solutions, such as the high solubility of metals, etc.
There are also recognized disadvantages associated with current wet chemical surface preparation methods. For example, to minimize the problems associated with contamination, chemicals are being replaced frequently, thereby generating large amounts of chemical wastes. Stringent environmental regulations decrease the options for disposing of these chemicals. Legislation like the Clean Air and Water Acts, Pollution Prevention Act Resource Conservation and Recovery Act, Source Reduction and Hazardous Waste Management Review Act, Hazardous and Solid Waste Amendment etc., prohibit easy off-site disposal of these chemicals. Difficulty in disposal makes the option of simply introducing a steady stream of unused, high purity chemicals problematical and contributes to escalating cost, etc. Some of these environmental, safety and health issues are being addressed by reducing the concentration of liquid based chemicals, process improvements and equipment improvements, as well as the recycling of chemicals. There is an ongoing need for new, improved and innovative processes that address many of these challenges.
One resolution that has been tried to control contamination in wet chemical processes is to have the chemical generation system located very close to the wet station, but still beyond the boundaries of the cleanroom area itself. This proximity to the wet station for the creation of the chemicals is believed to improve their purity.
Closer to the wet chemical processes, there are other steps that can be examined and changed, for example, how often the tank in the wet benches or wet station must be dumped and refilled, the amount of recirculation that should be tolerated and other protocols that should be considered, such as the injection of ozone to help keep the bath fresh. Another very important factor to be considered is whether or not using filters will positively affect the process, and if so where in the processes should those filters be utilized. Generally, filters are not used with point-of-use (POU) chemical delivery systems. These delivery systems use clean piping and valves that minimize the generation of particles and make POU filtration unnecessary, the belief being that point-of-use filtration is not necessary if the wet bench is fed by a properly designed bulk chemical distribution system. On the other hand, filters are widely used with recirculated bath applications. Here, particles are generated by the processing of wafers and must be removed to prevent their adherence to the wafer surface.
What benefit filters ultimately confer is a point of controversy in many semiconductor processes. Filtration by its nature, can remove particles that are present in the chemical, but cannot ensure purity. A filter can function to prevent introduction of further extractable species (ionic or organic) to that chemical, but cannot upgrade the starting level of purity for a chemical substance passing through the filter. Under some operating conditions, filters can actually add to the particulate contamination. For example bacteria trapped in a filter can actually break up in small pieces and be jettisoned through to the clean side of the filter, increasing rather than decreasing the amount of particulate contamination present.
Overall, the use of filters and POU chemical systems help reduce the problem of particle contamination in the liquids used in semiconductor fabrication. There is an existing need for new, improved processes that address the issue of contamination by gaseous species in processing liquids.